Two Optiplex 7050 PCs have stopped seeing NVMe drives in M.2 slot

I have six Dell Optiplex 7050 PCs. All were supplied at the same time and were fitted with a 1TB HDD as standard, but I installed Samsumg EVO NVMe drives into the M.2 slots for the Windows 10 OS. 

All systems have been working fine for years, but two that haven't been used for several months are no longer booting and the NVMe drives are no longer visible in the BIOS.  I have done the following:  

1.  Replaced CMOS backup cells

2.  Tried all manner of settings changes in BIOS  

3.  Tested the NVMe drives in another 7050 and they're both fine

4.  Flashed the BIOS anyway even though all PCs were already on the latest version 1.27

What possible reason could there be for these two PCs to be suddenly not seeing the drives in the M.2 slots?  

The obvious culprit was the CMOS battery.  The date was still correct, but I replaced it anyway.  It was down to 2.8V so I was hopeful that this would fix the issue, but it didn't.  

Restoring BIOS settings from 'Last good known', 'Factory' and 'Defaults' hasn't worked even though the settings on a working PC work fine with the defaults. 

I've also tried all manner of settings including disabling secure boot/enabling legacy, setting SATA to Raid, Disabled etc.  Not that I have any reason to think these will be the issue since the working PCs are fine with the BIOS defaults, but I'm completely stumped and just started trying different settings as a last resort.  

The drives themselves are fine as they work okay in one of the other PCs, but these show the drive when present in the M.2 slot and the problem PCs don't. 

What could possibly cause two working 7050s to stop seeing the drives in their M.2 slots after being left alone for several months?  

Of course I've made sure that the drives are properly seated in the M.2 connectors and that the edge connection strips are clean and undamaged.  

Is there a way I can print out or list all the BIOS settings to do a thorough comparison between the good and bad PCs?  

Any other help/suggestions would be welcome.